Frankly, I don't think that your RFI problems are due to your speaker wires. That is assuming that your rear channel speakers are NOT self powered. Those wires are carrying voltage levels many many times higher than the millivolt levels that might be generated by RFI. I think that you problem may be inside your receiver. If so, all the shielding in the world won't cure your RFI issue until you address in at it's source. One simple way to be sure is to disconnect the speaker wires at the receiver and see if you hear anything from the speakers.

BTW, you don't state exactly what your hearing in your rear channels. If it's a hum, that may be a problem with grounding or possibly your power conditioner has some issues. If you have your receiver sitting on top of your power conditioner, the first thing that I would try is seperating the 2 components. That power conditioner may be broadcasting a low level of RFI and seperating it from your receiver may solve all of your problems, 2 or 3 feet should be enough. Second, look at all of your interconnects and try and route them so they are well away from any power cords and that power conditioner. Third, try and have the complete system powered by just one outlet. Sometimes connecting to multiple outlets can cause a ground hum because one of the outlets may have a ground that is at a different level that another outlet. BTW, this is a potentially hazardous condition so if you find powering everything from one outlet solves the problem, call an electrician and have him check the grounding for every outlet in your house because a failed ground could lead to a fire or electrocution.

Finally, if your receiver is getting up in years, it might be time to think about a new one. I had to replace my integrated amplifier because it had started to pick up police cars prowling the neighborhood. Getting it fixed would have probably cost more than a new amp so I just tossed it and got a new amplifier.

I don't know what happened to my earlier post to this thread, but here's a replacement. Speaker level cables seldom have any RF interference problems. Only if the speaker cables are routed very close to and running in parallel with some AC lines at some point would I expect a RF problem in speaker cables. You can check for cable problems by disconnecting the cables to the rear speakers at the receiver, but you will have to have the cables plugged into another speaker output and the receiver powered up to check for cable problems. If the cables are simply unplugged from the speaker outputs you don't have a circuit which would pick up any noise. RF problems are typically related to a tuned circuit reacting at a specific frequency.

Plug your surround speaker cables into the front speaker outputs and listen for RFI coming through the speakers. If you hear nothing while playing music at a typical volume, then your cables are not your problem. While you're at the back of the receievr, run your front speakers from the surround speaker outputs to check for RF pickup within the receiver.

As suggested you haven't given us much to go on. Has the problem always been present or has it crept up or suddenly appeared. Each of these situations would suggest other problems. You might simply have an oxidized RCA connector running to your rear channel inputs which is acting as a diode and picking up RF. While you're behind the amp doing the suggested reconnections, do some basic house keeping and clean all your breakable contacts with a small amount of Pro Gold contact cleaner and dress your cables to keep speaker lines away from power cables and line level cables running separate from both of those two. Have as much space between each group of cables as possible and only let other cables cross AC lines at a right angle to the AC cable. After you've done some reconnection and some cleaning, let us know if the problem still exists.

First off, wood isn't very effective at blocking RFI. If it were, none of use would be able to listen to a tabletop radio without hooking it up to an external antenna. So, get that power conditioner AWAY from your receiver. Like I said, 2 or 3 feet should be sufficient because if your power conditioner is acting like a relay transmitter it will be at a very low output.

Second, as suggested by Jan, clean all the contacts on the rear of your receiver. However, to eliminate the possibility that one of you source components is passing along this stray RFI, try this. Disconnect every single input source AT THE RECEIVER, turn on your receiver, and then listen for that radio station when nothing is feeding your receiver. If, when nothing is connected, you still hear that country station you can be pretty sure the receiver is the fault. If you don't hear anything, then one of your sources is passing the RFI to your receiver. You can determine which one it is by connecting your sources one at a time. As soon as you have identified the problem source, try swapping the interconnect cables with those from a "neutral" source. That will allow you to determine if you have a noisy source component or a bad set of interconnects.

Third, since you have had this problem from day one with your receiver, it's possible that you received a defective unit. Even with a name brand like NAD it's still possible to get a bad unit occasionally. That is why they have a warranty, to protect the consumer on the rare occasion that a defective unit snuck past quality control. In this case the cause could be as simple as just one capaciter that isn't up to specification. I would suggest that you contact your dealer ASAP because your warranty may be about to expire.

I hope this helps. You'll notice that I didn't pay any attention to your speaker cables. There is a very simple reason for this. RFI as you have described requires a LOT of amplification before it can become audible thru room speakers. Since the speaker cables are on the output side of the amplification chain I just don't think it's possible for RFI picked up by your speaker cables to become audible.